Electron emission technology exists in many forms today. Hot cathode ray tubes (CRT), where electrons are produces as a result of thermal emission from hot cathode heated by electrical current, are prevalent in many displays such as televisions (TV) and computer monitors. Electron emission also plays a critical role in devices such as x-ray machines and electron microscopes. Miniature cold cathodes may be used for integrated circuits and flat display units. In addition, high-current density emitted electrons may be used to sputter or melt some materials.
In general, two types of electron emitters exist—“hot” and “cold” cathode emitters. The “hot” cathodes are based on thermal electron emission from surface heated by electric current. The cold cathodes can be subdivided into two different types: type A and B. The emitters of type A are based on the field emission effect (field-emission cathodes). The emitters of type B are the p-n cathodes using the emission of non-equilibrium electrons generated by injection or avalanche electrical breakdown processes.
Both types of emitters have drawbacks, which make them virtually impractical. For type A emitters (field emission type), one of the main drawbacks is the very short lifetime of such emitters. For example, the type A emitters may be operational for just hours, and perhaps even as short as minutes. In the cold field-emission cathodes (type A), electrons are extracted from the surface of a metal electrode by a strong electric field in vacuum. The field cathodes have a short lifetime at large emitted currents, which are needed in recording devices and other applications.
With reference to FIG. 1A, operation of type A emitters will be described. FIG. 1A illustrates a typical energy diagram for a metallic surface illustrating a concept of a work function of a metal. As shown, a material, in this instance a metal, is on the left and a vacuum region is on the right. EF represents a Fermi level of the metal. The work function of the metal ΦM is the energy required to move a single electron from the Fermi level in the metal into vacuum. Thus, the work function ΦM is the difference between Vac and EF. The work function ΦM for metal is typically between 4-5 electron volts (eV).
In very strong external field the energy diagram changes, and behaves as a triangular potential barrier for the electrons (FIG. 1A, dashed line). When the external field F increases, the barrier width decreases and the tunneling probability for electrons rapidly increases. The transparency of such a barrier is       D    =          exp      ⁡              [                  -                                    4              ⁢                              Φ                M                                  3                  /                  2                                            ⁢                                                2                  ⁢                  m                                                                    3              ⁢              q              ⁢                                                           ⁢              h              ⁢                                                           ⁢              F                                      ]              ,where F the electric field, q and m are the electron charge and mass. Transparency represents the probability of electron tunneling through the barrier. For current densities j=1-100 A/cm2 (amperes per square centimeter) the corresponding field would be F>107 V/cm.
In such strong fields, the ions, which are always present in a vacuum region in actual devices, acquire the energy over 103 eV in the vacuum region on the order of one micron or larger. Ions with such strong energies collide with the emitter surface leading to absorption of the ions and erosion of the emitter surface. The ion absorption and erosion typically limits the lifetime of type A emitters to a few hours of operation or even to a few minutes. Damage to cathodes in systems with the fields of similar strength has been studied in great detail and is rather dramatic.
For type B emitters (injection/avalanche type), one of the main drawbacks is that the efficiency is very small. In other words, the ratio of emitted current to the total current in the circuit is very low, usually much less than 1%. The cathode of type B based either on p-n junctions, or semiconductor-metal (S-M) junction including TiO2 or porous Si, or the avalanche electrical breakdown need an “internal” bias, applied to p-n junction or S-M junction.
Alternatively, there have been suggestions to use the electrical breakdown processes to manufacture the cold emitters from Si. These types of avalanche emitters are based on emission of very hot electrons (with energies of the order of a few electron volts) accelerated by very strong electric field in the avalanche regime. As a result, they also have a disadvantage that the emitted current density of the hot electrons is very small.
Attempts have been made to increase the current density by depositing cesium (Cs) on semiconductor surface to use a negative electron affinity (NEA) effect. FIG. 1B illustrates the concept of NEA. As shown, a material, a p-type semiconductor in this instance, is on the left and a vacuum region is on the right. EC represents a conduction band of the metal. Note that the NEA effect corresponds to a situation when the bottom of the conduction band EC lies above the vacuum level Vac. One earlier p-n cathode of this type combined a silicon, or gallium arsenide avalanche region, with cesium metallic layer from where the emission took place (GaAs/Cs or GaP/Cs structures). However, Cs is a very reactive and volatile element. Thus, the GaAs and GaP emitters with Cs are not stable at high current densities.
In short, cold emitters with both high current emission and stability were not possible with previous designs.